In conventional drilling, a cutting tool rotates around its own axis while feeding forward into the material being drilled. The cutting edges perform best at the outside edges while the center of the drill essentially does not cut, but merely pushes its way through the material. This generates a lot of heat and thrust force which requires additional power from the drilling machine. When the drill reaches the other side of the material being drilled, the drill tip bursts through creating ragged edges that spread out to the final sized hole, leaving an exit burr around the hole. Exit burrs are unacceptable in aerospace assembly, and in many other industries, as they create stress risers and fatigue cracks in the structure.
Orbital drilling has been used in an effort to avoid many of the problems often associated with conventional drilling. With orbital drilling, the cutter rotates around its own axis but the cutter spindle is offset from the centerline of the hole by the orbital axis of the machine. This dual rotating axis approach can be described as a spiral milling method of generating a hole. The cutting edges travel at very high speed and create very small chips that are vacuumed away. No lubrication is required and burrs are typically not generated as the cutter exits the material. Moreover, the same holes can be generated using much less thrust force and torque than would be needed from a conventional drilling machine.
In the airline industry, amongst other industries, work-pieces comprised of multiple layers of material are often utilized. These layers of material often have varying hardness levels. When the outermost layer of material of the work-piece is extremely hard, such as Titanium, it is difficult to accurately and efficiently drill a hole through the multiple layers of material, even utilizing orbital hole drilling. During such an orbital drilling process, damage to the cutting tool may occur, the hole may end up misaligned, the drilling process may take an extended time, and the drilled hole may end up having sharp, undesired burrs.
A method of orbital drilling a hole, in a work-piece having multiple layers, is needed to substantially avoid one or more of the problems associated with having to simultaneously drill through a hard outer layer while drilling the hole in the work-piece.